Carbon fibers are generally produced by carbonizing or graphitizing carbon fiber precursor material fibers. Conventional carbon fiber precursors may be formed from polyacrylonitrile (PAN), petroleum pitch, or rayon precursors, by way of example. Carbon fibers and graphite fibers are made and heat-treated at different temperatures and thus each has different carbon content. Typically a carbon fiber is fiber that has at least about 90% by weight carbon.
Carbon fibers are used as a light-weight reinforcement phase to make high-strength light-weight polymeric composite materials. The carbon fibers may be continuous filaments that may be thousands of micrometers (μm) or millimeters (mm) in length. A group of continuous carbon fibers are often categorized as a bundle of continuous carbon fiber filaments. Carbon fiber “tow” is usually designated as a number of filaments in thousands (designated by K after the respective tow number). Alternatively, carbon fiber bundles may be chopped or milled and thus form short segments of carbon fibers (filaments or bundles) typically having a mean fiber length between 50 μm and 50 mm (about 1.97 inches). While composites incorporating carbon fibers are all light-weight and high-strength, composites incorporating continuous carbon fiber filaments have especially high strength as compared to composites incorporating chopped or milled carbon fibers. By way of non-limiting example, a representative unidirectional continuous carbon fiber filament when incorporated into a composite has an ultrahigh ultimate tensile strength of about 1,500 to 2,000 MPa, while chopped carbon fibers have an ultimate tensile strength of about 200 MPa to 500 MPa.
While the ultrahigh strengths are highly desirable in certain applications, one technical challenge in using continuous carbon fibers in composites is the lack of flowability and formability, because composite pre-pregs incorporating continuous carbon fibers can be too stiff having high resistance to flow. Such inflexibility and rigidity translates to poor moldability, making it difficult to form three-dimensional shapes from composites having continuous carbon fibers. It would be desirable to form continuous carbon fiber composites having greater flexibility, higher flowability, and thus greater moldability with the capability of readily forming complex and three-dimensionally shaped components with ultrahigh-strengths.